1. Field of the Invention
This invention relates to coating compositions which are especially useful for clear coating over a colored base coat.
2. Description of Related Art
U.S. Pat. No. 5,279,862 discloses a clear coating composition which can be used as the clear coat of a motorized vehicle clear coat/color coat finish. The clear coating composition has a film-forming binder and volatile organic liquid carrier, the binder including hydroxy functional acrylic polymer and organic polyisocyanate and is characterized by rapid curing to form a finish that is tack free and can be buffed as soon as 3-4 hours after application, resulting in increased productivity of the paint shop.
Even more rapid curing of the clear coat is desired for further increase in paint shop productivity. A catalyst for the hydroxy-isocyanate crosslinking reaction is present in the clear coating composition, typically an organo tin compound. When the amount of catalyst is increased to speed-up the cure, other problems arise, including decreased potlife and reduced coating quality. In the latter case, the faster cure entraps liquid carrier within the dried clear coating, causing the coating to have poor gloss and distinctness of image.
There are other indicia of increased productivity, e.g. how soon after application the film coating dries sufficiently to be dust-free, so that the painted article (vehicle) can be moved from the paint booth, to make room for the next vehicle to be painted. The vehicle can be moved outside the paint shop, i.e. into the open air, only after the film coating has dried further so as to be free of water spotting damage.
The present invention provides a coating composition which forms film coatings which provide improved productivity as determined by one or more of the indicia of rapidly becoming dust-free and water spot resistant, and/or rapid curing sufficiently to be buffed, all occuring at ambient temperature (20xc2x0 C.).
The coating composition of the present invention contains a film forming binder and a volatile organic liquid carrier binder, wherein the binder contains
(A) hydroxyl-containing acrylic polymer and polyhydroxyl-tertiary amine, at least two hydroxyl groups of said amine being indirectly bonded to a nitrogen atom of said amine through a chain containing at least two carbon atoms, and
(B) organic polyisocyanate,
the ratio of equivalents of isocyanate in (B) per equivalent of hydroxyl groups in (A) being in the range of 0.5/1 to 3.0/1, and
(C) an effective amount of catalyst to cure said composition.
In the broadest sense, the present invention comprises component (A) by itself, preferably also contained in a volatile organic liquid carrier, the combination of the hydroxyl-containing acrylic polymer and polyhydroxyl-tertiary amine also constituting a film-forming binder. Components (A) and (B) are packaged separately and are combined just prior to application, because component (B) crosslinks the combined components. Component (C) speeds up the crosslinking reaction and can be provided to the crosslinking reaction either as part of (A), (B), or as a separate component. In any event, the pot-life of the combined components is sufficient to enable the combined components to be applied, typically by spraying, onto the substrate to be coated, typically an vehicle body part, including the entire vehicle body.
The curing of the composition after it is applied to form the film coating occurs by the isocyanate groups of (B) reacting with the hydroxyl groups of both the acrylic polymer and the polyhydroxyl-tertiary amine of (A) to form urethane linkages, whereby the cured coating film is a polyurethane. The polyhydroxyl-tertiary amine both speeds up the curing reaction, as does the catalyst (C), and becomes part of the crosslinked structure by the reaction of its hydroxyl groups with the isocyanate groups. Thus, the polyhydroxyl-tertiary amine is present in an effective amount to increase the crosslinking reaction rate during curing of the composition. Preferably, the amount of said acrylic polymer is 40-99 wt % and the amount of polyhydroxyl-tertiary amine is 1-60 wt %, based on the total weight of (A).
Film coatings formed from compositions of the present invention typically become dust-free within 10 min and even within 5 min, free of water spot damage within 30 min, and can be buffed in less than three hours and possibly as early as one hour after application, all with ambient temperature drying and cure, without sacrifice in either the ease of applying the coating composition or the ultimate quality of the clear coat. Of course, the film coating becomes tack-free prior to becoming buffable.
Thus, the coating compositions of the present invention are highly useful for repairing a clearcoat/colorcoat finish of a vehicle using the coating composition as a refinish clearcoat, which process allows the vehicle to be moved outside and the finish to be sanded (wet or dry), buffed or polished, if necessary, to remove minor imperfections and enhance gloss within a short period of time after application. This greatly improves the productivity of a refinish operation by allowing more vehicles to be processed in the same or in less time.
The coating composition of this invention is a low VOC (volatile organic content) composition that is particularly suited for use as a clearcoat in automotive refinishing. The composition contains a film forming binder and an organic liquid carrier which is usually a solvent for the binder. Since the invention is directed to a low VOC composition, the amount of organic solvent used in the liquid carrier portion results in the composition having a VOC content of less than 0.6 kilograms per liter (5 pounds per gallon) and preferably in the range of about 0.25-0.53 kilograms (2.1-4.4 pounds per gallon) of organic solvent per liter of the composition, as determined under the procedure provided in ASTM D-3960. This usually translates to a film forming binder content (components (A)+(B)+(C)) of about 25-90% by weight and an organic liquid carrier content of about 10-75% by weight, preferably about 30-55% by weight binder and 45-70% by weight carrier. Component (A), by itself can have the same solids content in organic liquid carrier, with or without the presence of component (C) in component (A). xe2x80x9cSolids contentxe2x80x9d as used herein refers to the film-forming binder content of the composition, i.e. although the binder is in solution in the carrier, upon evaporation of the carrier, solid coating film of the binder remains.
The hydroxyl functional acrylic polymer used in the hydroxyl component of the binder is prepared by conventional solution polymerization techniques in which monomers, solvents and polymerization catalyst are charged into a conventional polymerization reactor and heated to about 60-200xc2x0 C. for about 0.5-6 hours to form a polymer having a weight average molecular weight (Mw) of preferably about 2,000-13,000, more preferably about 3,000-11,000.
All molecular weights disclosed herein are determined by GPC (gel permeation chromatography) using polymethyl methacrylate standard, unless otherwise noted.
The acrylic polymer thus formed also has a glass transition temperature (Tg) generally of at least 20xc2x0 C. and preferably about 40-80xc2x0 C.
All glass transition temperatures disclosed herein are determined by DSC (differential scanning calorimetry).
Typically useful polymerization catalysts are azo type catalysts such as azo-bis-isobutyronitrile, 1,1xe2x80x2-azo-bis(cyanocylohexane), acetates such as t-butyl peracetate, peroxides such as di-t-butyl peroxide, benzoates such as t-butyl perbenzoate, octoates such as t-butyl peroctoate and the like.
Typical solvents that can be used are ketones such as methyl amyl ketone, methyl isobutyl ketone, methyl ethyl ketone, aromatic hydrocarbons such as toluene, xylene, alkylene carbonates such as propylene carbonate, n-methyl pyrrolidone, ethers, ester, such as butyl acetate, and mixtures of any of the above.
The hydroxyl functional acrylic polymer is preferably composed of a mixture of monomers, predominantly (meth)acrylic which copolymerize together to provide the application and cured coating film characteristics desired polymerized. In accordance with the present invention it is important that the acrylic polymer also contain hydroxyl groups provided by one or more comonomers used to form the acrylic polymer. The preferred mixture of comonomers is styrene, a methacrylate which is either methyl methacrylate, isobomyl methacrylate, cyclohexyl methacrylate or a mixture of these monomers, a second methacrylate monomer which is either n-butyl methacrylate, isobutyl methacrylate or ethyl hexyl methacrylate or a mixture of these monomers and a hydroxy alkyl methacrylate or acrylate that has 1-8 carbon atoms in the alkyl group such as hydroxy ethyl methacrylate, hydroxy propyl methacrylate, hydroxy butyl methacrylate, hydroxy ethyl acrylate, hydroxy propyl acrylate, hydroxy butyl acrylate and the like.
A preferred acrylic polymer contains about 5-30% by weight styrene, 1-50% by weight of the methacrylate, 30-60% by weight of the second methacrylate and 10-40% by weight of the hydroxy alkyl methacrylate. The total percentage of monomers in the polymer equal 100%. One particularly preferred acrylic polymer contains the following constituents in the above percentage ranges: styrene, methyl methacrylate, isobutyl methacrylate and hydroxy ethyl methacrylate. Another preferred acrylic polymer contains the following constituents in the above percentage ranges: styrene, isobornyl methacrylate, ethyl hexyl methacrylate, hydroxy ethyl methacrylate and hydroxy propyl methacrylate. Still another preferred acrylic polymer contains the following constituents in the above percentages: styrene, methyl methacrylate, isobornyl methacrylate, ethyl hexyl methacrylate, isobutyl methacrylate, and hydroxy ethyl methacrylate. Most preferably, compatible blends of two or more of the above acrylic polymers are used.
Optionally, the acrylic polymer can contain about 0.5-2% by weight, based on the weight of acrylic polymer, of acrylamide or methacrylamide such as n-tertiary butyl acrylamide or methacrylamide, copolymerized with the acrylic polymer.
The polyhydroxyl-tertiary amine is present in (A) as a mixture with the acrylic polymer in solution in the liquid carrier. Its essential components are the presence of at least one tertiary amine nitrogen atom and a plurality of hydroxyl groups, with at least two of the hydroxyl groups being bonded to at least one of the nitrogen atoms via a bifunctional group which contains at least two carbon atoms, i.e. the xe2x80x94OH substitution is no closer than beta to the nitrogen atom. The bifunctional group can be an aliphatic group preferably containing 2 to 12 carbon atoms. When the tertiary amine has two nitrogen atoms, preferably at least one xe2x80x94OH group is indirectly substituted onto each nitrogen atom as described above. The polyhydroxyl-tertiary amine is non-reactive with the acrylic polymer and can be used in the composition of the present invention as a single polyhydroxyl-tertiary amine or as a mixture of different polyhydroxyl-tertiary amines. Polyhydroxyl-tertiary amines that can be used in the composition of the present invention include those represented by the formula 
wherein R is alkylene or oxyalkylene containing 0 to 6 carbon atoms and X and Y are independently R1H, wherein R1 is xe2x80x94(CH2CH2O)nxe2x80x94, xe2x80x94(CH2C(CH3)HO)nxe2x80x94, xe2x80x94(CH2C([CH2]mCH3)HO)nxe2x80x94 wherein m is an integer of 1-5, or (CH2C(C6H11)HO)nxe2x80x94, or any combination thereof, wherein n is an integer of 1-3, and
A is Rxe2x80x94X, Rxe2x80x94Y, R2 or Z,
wherein R2 is an alkyl group containing 1-20 carbon atoms and
Z is 
wherein R3 is an alkylene group containing 1 to 10 carbon atoms or a cycloalkylene or arylene group containing 3 to 21 carbon atoms, with the proviso that at least two, preferably at least three, xe2x80x94OH groups (provided by R1H) are present.
Preferably, R when present is alkylene or oxyalkylene containing 0 to 4 carbon atoms, R1 is xe2x80x94(CH2CH2O)nxe2x80x94, xe2x80x94(CH2C(CH3)OH)n, or CH2C(CH2CH3)HO)nxe2x80x94, wherein n is 1 or 2, R2 is an alkyl group containing 1 to 20 carbon atoms, and R3 is an alkylene group containing 1 to 6 carbon atoms or a cycloalkylene or arylene group containing 3 to 21 carbon atoms. The combination of R and R1 form one embodiment of aliphatic group indirectly connecting the xe2x80x94OH group to the nitrogen atom.
Examples of polyhydroxyl-tertiary amines include simple compounds such as N,N-diethanol alkyl amine, triethanol amine and more complicated compounds which can be considered as oligomers, such as the Ethomeen(copyright) (one tertiary amine nitrogen atom) and Ethoduomeen(copyright) (two tertiary amine nitrogen atoms) compounds available from Akzo Nobel. Examples of these compounds in which only one tertiary amine nitrogen is present are represented by the formula 
Wherein R2, R, X, and Y have the same meaning as described above and wherein R2 preferably has 8 to 20 carbon atoms. Examples of group R2 are tallow, oleyl, coco, and soya. A preferred group of compounds are the diethoxylates characterized by the formula 
Wherein R2 contains 8-20 carbon atoms. Examples of compounds containing two tertiary amine nitrogen atoms include the Ethoduomeens such as the compound having the formula 
and the compounds having the formula 
Wherein m is an integer independently selected from the group 0, 1, or 2, and o is an integer of from 1 to 4.
In the Ethomeen(copyright) and Ethoduomeen(copyright) compounds containing the R2 group, such group is a mixture of alkyl groups as shown in the following Table A.
Examples of other useful polyhydroxyl-tertiary amines containing two tertiary amine nitrogen atoms include compounds having the formula: 
wherein R1 is xe2x80x94(CH2CH2O)nxe2x80x94, xe2x80x94(CH2C(CE3)OH)nxe2x80x94, or xe2x80x94(CH2C(CH2CH3)HO)nxe2x80x94, n is 1 or 2, and R3 is a cycloalkylene radical from the group of: 
The combination of the hydroxyl-containing acrylic polymer and the polyhydroxyl-tertiary amine (component (A)) crosslinked with the polyisocyanate (component (B)) to be described hereinafter produces a clear, tough glossy film coating. The proportions of the acrylic polymer and polyhydroxyl-tertiary amine required to produce this result, along with quick curing will depend on the particular acrylic polymer and polyhydroxyl-tertiary amine selected, and to some extent on the particular polyisocyanate selected. Preferably, however, an effective amount of the polyhydroxyl-tertiary amine will be present to reduce the curing time so that water spot damage does not occur after one hour after application of the coating, followed by drying at ambient temperature (20xc2x0 C.). Typically, the amount of polyhydroxyl tertiary amine needed to achieve this goal will be from 1 to 20 wt % of component (A).
Component (A) can further contain a hydroxyl-terminated polyester such as that having a weight average molecular weight (Mw) not exceeding about 3,000 (oligomer), preferably about 200-2,000, and a polydispersity (Mw divided by Mn) of less than about 1.7.
Typically useful such oligomers include caprolactone oligomers containing terminal hydroxyl groups which may be prepared by initiating the polymerization of caprolactone with a cyclic polyol, particularly a cycloaliphatic polyol, in the presence of a tin catalysts via conventional solution polymerization techniques. Such caprolactone oligomers are well known and described at length in Anderson et al. U.S. Pat. No. 5,354,797, issued Oct. 11, 1994. Epsilon(xcex5)-caprolactone is typically employed as the caprolactone component in a 1/1 to 5/1 molar ratio with a cycloaliphatic diol. Typically useful cycloaliphatic polyol monomers include 1,4-cyclohexanediol, 1,4-cyclohexane dimethanol, and 2,2xe2x80x2-bis(4-hydroxycyclohexyl) propane. Preferred caprolactone oligomers are formed from caprolactone and 1,4-cyclohexanedimethanol reacted in a molar ratio of 2/1 to 3/1.
Other useful oligomers include alkylene oxide polyester oligomers containing terminal hydroxyl groups which may be made by reacting stoichiometric amounts of a cycloaliphatic monomeric anhydride with a linear or branched polyol in solution at elevated temperatures in the presence of a tin catalyst using standard techniques and then capping the acid oligomers so formed with monofunctional epoxies, particularly alkylene oxide, under pressure above atmospheric but not exceeding about 200 psi and at temperatures of 60-200xc2x0 C. for 1 to 24 hours. Such alkylene oxide oligomers are well known and described at length in Barsotti et al. PCT Application No. US98/23337, published May 14, 1999.
Cycloaliphatic anhydride monomers such as hexahydrophthalic anhydride and methyl hexahydrophthalic anhydride are typically employed in the alkylene oxide oligomers above. Aliphatic or aromatic anhydrides, such as succinic anhydride or phthalic anhydride may also be used in conjunction with the anhydrides described above. Typically useful linear or branched polyols include, hexanediol, 1,4-cyclohexane dimethanol, trimethylol propane, and pentaerythritol. Useful monofunctional epoxies include alkylene oxides of 2 to 12 carbon atoms. Ethylene, propylene and butylene oxides are preferred although ethylene oxide is most preferred. Other epoxies, such as xe2x80x9cCarduraxe2x80x9d E-5 or xe2x80x9cCarduraxe2x80x9d E-10 glycidyl ether, supplied by Exxon Chemicals, may be used in conjunction with the monofunctional epoxies described above. Particularly preferred alkylene oxide oligomers are formed from methyl hexahydrophthalic anhydride; either 1,4-cyclohexanedimethanol, trimethylol propane, or pentaerythritol; and ethylene oxide reacted in stoichiometric amounts.
Compatible blends of any of the aforementioned hydroxyl-terminated polyesters can be used as well in the hydroxyl component (A) of the binder. Generally, 0 to 39 wt %, based on the total weight of component (A) of the hydroxyl-terminated polyester will be present, and preferably the amount will be 1 to 20 wt %.
The polyisocyanate component of the binder includes an organic polyisocyanate as the crosslinking agent. The organic polyisocyanate can be a single polyisocyanate or a blend of different polyisocyanates.
Any of the conventional aromatic, aliphatic, cycloaliphatic diisocyanates, trifunctional isocyanates and isocyanate functional addition compounds of a polyol and a diisocyanate may be used as or in the polyisocyanate component (B).
Typically useful diisocyanates are 1,6-hexamethylene diisocyanate, isophorone diisocyanate, 4,4xe2x80x2-biphenylene diisocyanate, toluene diisocyanate, bis cyclohexyl diisocyanate, tetramethylene xylene diisocyanate, ethyl ethylene diisocyanate, 2,3-dimethyl ethylene diisocyanate, 1-methyltrimethylene diisocyanate, 1,3-cyclopentylene diisocyanate, 1,4-cyclohexylene diisocyanate, 1,3-phenylene diisocyanate, 1,5-naphthalene diisocyanate, bis(4-isocyanatocyclohexyl)-methane, 4,4xe2x80x2-diisocyanatodiphenyl ether and the like.
Typical trifunctional isocyanates that can be used are triphenylmethane triisocyanate, 1,3,5-benzene triisocyanate, 2,4,6-toluene triisocyanate and the like. Trimers of other diisocyanates also can be used such as the trimer of hexamethylene diisocyanate (HDI) which is sold under the tradename xe2x80x9cDesmodurxe2x80x9d N-3300 or N-3390 or xe2x80x9cTolonatexe2x80x9d HDT or HDT-LV. Trimer of isophorone diisocyanate (IPDI) can also be used. In forming the trimer from isophorone diisocyanate, one of the isocyanate groups forms an isocyanurate group; the resultant trimer, however, has three isocyanate groups. Typically useful IPDI trimers are sold under the tradenames xe2x80x9cDesmodurxe2x80x9d Z-4470 BA or SN/BA or SN or MPA/X. The IPDI trimer offers the resulting coating improved hardness on curing.
In the present invention, the polyisocyanate component (B) contains 0%, but preferably at least 3% up to about 50% by weight, more preferably about 5-30% by weight, of the IPDI trimer, based on the weight of component (B). Excessive IPDI trimer tends to cause the film coating to become too brittle, whereby the coating film will crack over time.
Isocyanate functional addition compounds can also be used that are formed from an organic polyisocyanate and a polyol (the reaction forming the addition compound uses up the xe2x95x90OH groups of the polyol). Any of the aforementioned polyisocyanates can be used with a polyol to form an addition compound. Polyols such as trimethylol alkanes like trimethylol propane or ethane can be used. One useful adduct is the reaction product of tetramethylxylidene diisocyanate and trimethylol propane and is sold under the tradename xe2x80x9cCythanexe2x80x9d 3160.
One particularly preferred polyisocyanate crosslinking component comprises a mixture of about 5-45% by weight IPDI trimer and about 55-95% by weight HDI trimer, based on the total weight of component (B). The preferred amount of IPDI trimer for use in combination with HDI trimer is 5 to 25 wt %. It is generally preferred to employ an HDI trimer in combination with the IPDI trimer to retain flexibility in the coating film.
The hydroxyl and polyisocyanate components (A) and (B), respectively are preferably employed in an equivalent ratio of isocyanate groups to hydroxyl groups of 0.8/1 to 2.0/1. The coating composition also contains a sufficient amount of catalyst to cure the composition at ambient temperature. A combination of certain catalysts is preferred when IPDI trimer is present, to accelerate the curing rate of IPDI trimer at room temperature to achieve the high film hardness offered by IPDI in a relatively short period of time, with little or no pot life reductions or die-back in the coating film formed therefrom. Even at these accelerated curing rates, the coating compositions remains processable for at least 30 minutes at ambient temperatures which provides enough time to complete the refinish job without the need for viscosity adjustments, and the high gloss coating film formed therefrom shows virtually no signs of dying back to a dull fuzzy finish over time.
The catalyst comprises at least one organotin tin compound, optionally at least one tertiary amine, and optionally, at least one organic acid in amounts described below (catalyst system).
Typically useful organotin compounds include organotin carboxylates, particularly dialkyl tin carboxylates of aliphatic carboxylic acids, such as dibutyl tin dilaurate (DBTDL), dibutyl tin dioctoate, dibutyl tin diacetate, and the like. Although not preferred, any of the other customary organotin or organometallic (Zn, Cd, Pb) catalysts could also be used. The amount of organotin catalyst employed in the coating composition can vary considerably depending on the specific binder system and the degree of initial hardness desired. However, it is critical that the coating composition contains enough organotin catalyst to cure the composition at ambient temperatures, while at the same time being insufficient to cause die-back.
Generally, about 0.005-0.2% by weight, based on the weight of the binder (components (A)+(B)+(C)), of organotin catalyst will be sufficient to impart the desired properties. It has been found that above the upper limit of 0.2%, the curing reaction is too fast and die-back results. Below about 0.005%, the curing reaction is too slow and insufficient hardness and poor mechanical properties develop. The organotin catalyst can be used by itself as the sole catalyst ingredient.
Typically useful tertiary amines useful as a co-catalyst in catalyst component (C), as distinguished from the polyhydroxyl-tertiary amine used in component (A), include tertiary aliphatic monoamines or diamines, particularly trialkylene diamines, such as triethylene diamine (DABCO), N-alkyl trimethylenediamine, such as N,N,Nxe2x80x2-trimethyl-Nxe2x80x2-tallow-1,3-diaminopropane, and the like; and trialkylamines such as tridodecylamine, trihexadecylamine, N,Nxe2x80x2-dimethylalkyl amine, such as N,Nxe2x80x2-dimethyldodecyl amine, and the like, all free of xe2x80x94OH groups. The alkyl or alkylene portions of these amines may be linear or branched and may contain 1-20 carbon atoms. Especially preferred are amines that contain at least 6 carbon atoms in at least one of their alkyl or alkylene portions to lower the hazing in humid conditions.
As with the amount of organotin compound, the amount of tertiary amine in the catalyst system employed in the coating composition can vary considerably, it being desired only that tertiary amine if present, be present in an amount which, together with the above, including component (A), will cause the composition to cure at ambient temperature within three hours, preferably within two hours. Generally, about 0.01-1% by weight, based on the weight of the binder, of tertiary amine will be sufficient to impart the desired properties. Above the upper limit of about 1%, the tertiary amine offers longer dust drying times and provides the film with insufficient hardness. Below about 0.01%, the catalytic effect is generally inadequate.
An organic acid is also included in the catalyst system for increased pot life. A pot life of at least 30 minutes at ambient temperatures is generally sufficient for completion of a refinish job. Typically useful acid catalysts are formic acid, acetic acid, proponic acid, butanoic acid, hexanoic acid, and any other aliphatic carboxylic acid, and the like. Generally, about 0.005-1%, based on the weight of the binder, of acid is employed.
It has been found that the three-component catalyst system described above offers a higher cure response than organotin, amine, or acid alone.
To improve weatherability of the composition about 0.1-10% by weight, based on the weight of the binder, of ultraviolet light stabilizers screeners, quenchers and antioxidants can be added. Typical ultraviolet light screeners and stabilizers include the following:
Benzophenones such as hydroxy dodecyloxy benzophenone, 2,4-dihydroxy benzophenone, hydroxy benzophenones containing sulfonic acid groups and the like.
Benzoates such as dibenzoate of diphenylol propane, tertiary butyl benzoate of diphenylol propane and the like.
Triazines such as 3,5-dialkyl-4-hydroxyphenyl derivatives of triazine, sulfur containing derivatives of dialkyl-4-hydroxy phenyl triazine, hydroxy phenyl-1,3,5-triazine and the like.
Triazoles such as 2-phenyl-4-(2,2xe2x80x2-dihydroxy benzoyl)-triazole, substituted benzotriazoles such as hydroxy-phenyltriazole and the like.
Hindered amines such as bis(1,2,2,6,6 entamethyl-4-piperidinyl sebacate), di[4(2,2,6,6,tetramethyl piperidinyl)]sebacate and the like and any mixtures of any of the above.
Generally, flow control agents are used in the composition in amounts of about 0.01-5% by weight, based on the weight of the binder, such as polyacrylic acid, polyalkylacrylates, polyether modified dimethyl polysiloxane copolymer and polyester modified polydimethyl siloxane.
When used as a clear coating, it may be desirable to use pigments in the clear coating composition which have the same refractive index as the dried coating. Typically, useful pigments have a particle size of about 0.015-50 microns and are used in a pigment to binder weight ratio of about 1:100 to 10:100 and are inorganic siliceous pigments such as silica pigment having a refractive index of about 1.4-1.6.
The coating composition of the present invention also contains the customary organic solvents in the organic liquid carrier portion. As previously described, the amount of organic solvent(s) added depends upon the desired binder level as well as the desired amount of VOC of the composition. Typical organic solvents consist of aromatic hydrocarbons, such as petroleum naphtha or xylenes; ketones, such as methyl amyl ketone, methyl isobutyl ketone, methyl ethyl ketone, or acetone; esters, such as butyl acetate or hexyl acetate; and glycol ether esters, such as propylene glycol monomethyl ether acetate. Examples of solvents which do not contribute to the VOC of the composition include methyl acetate, acetone, 1-chloro, 4-trifluoromethyl benzene, and potentially t-butyl acetate.
The coating composition of this invention is preferably prepared as a xe2x80x9ctwo-componentxe2x80x9d or xe2x80x9ctwo-packxe2x80x9d coating composition, wherein the two reactive binder components (A) and (B) are stored in separate containers, which are typically sealed. The catalyst (component (C), organic solvent, and usual other additives may be added to either or both the hydroxyl or crosslinking components, depending upon the intended use of the composition. However, these additives (except for some solvent) are preferably added to and stored in the same container with the hydroxyl component (A). The contents of the hydroxyl and isocyanate component containers are mixed in the desired NCO/OH ratio just prior to use to form the activated coating composition, which has a limited pot life. Mixing is usually accomplished simply by stirring at room temperature just before application. The coating composition is then applied as a layer of desired thickness on a substrate surface, such as an autobody part, including the entire autobody. After application, the layer dries and cures to form a coating on the substrate surface having the desired coating properties.
Generally, the coating composition of this invention is used as a clearcoat in automotive refinishing, but it should be understood that it can also be used as a clearcoat finish or can be pigmented with conventional pigments and used as a monocoat or as a basecoat in a clearcoat/colorcoat finish or refinish.
In the application of the coating composition as a clearcoat refinish to a vehicle part such as an automobile or a truck body or portion thereof, the basecoat which may be either a solvent based composition or a waterborne composition is first applied and then dried sufficiently to form a stable basecoat for the clear coat before the clearcoat is applied usually wet-on-wet by conventional spraying. Electrostatic spraying also may be used. In refinish applications, the composition is preferably dried and cured at ambient temperatures but can be forced dried and cured in paint booths equipped with heat sources at slightly elevated booth temperatures of, in general, about 30-100xc2x0 C., preferably about 35-65xc2x0 C., for a short time of about 3-30 minutes, preferably about 5-15 minutes. The coating so formed is typically about 0.5-5 mils (0.012 to 0.12 mm) thick.
In these refinish applications, in particular, the clearcoat of this invention has been found to greatly improve the productivity of a refinish operation. Through incorporation of a mixture of hydroxy-containing polyacrylic resin, polyhydroxyl-tertiary amine, polyisocyanate, preferably containing some IPDI trimer, and effective catalysts, the composition when used as a clearcoat dries and cures in a relatively short time after application to a dust free, water resistant, and sufficiently hard state for sanding (wet or dry) or buffing, unexpectedly with minimum pot life reductions and without die-back consequences, which allows the vehicle to be buffed, moved out of the way, and delivered to the customer on the same day of application, in comparison to the next day offered by conventional clear coat compositions. The composition of this invention, in particular, exhibits a pot life of at least 30 minutes at ambient temperature, dust free time within 10 minutes or less at ambient temperatures, and water spot free and wet sand or buff time within 3 hours, preferably within 2 hours, and even as soon as one hour, at ambient temperatures. The foregoing properties can be achieved much faster by curing the composition at slightly elevated temperatures of, in general, about 55-65xc2x0 C. peak substrate temperature for about 3-10 minutes, and preferably about 60xc2x0 C. for about 6 minutes, which remarkably allows the clear finish to be sanded or buffed immediately on cool down. Furthermore, the finish remains sandable or buffable for several days up to one week before it cures to a tough, hard durable exterior automotive finish.
The coating composition of this invention can be used to paint or repair a variety of substrates such as previously painted metal substrates, cold roll steel, steel coated with conventional primers such as electrodeposition primers, alkyd resin repair primers and the like, plastic type substrates such as polyester reinforced fiber glass, thermoplastic olefin (TPO), reaction injection molded urethanes and partially crystalline polyamides, as well as wood and aluminum substrates.